📚 DOCS: add sphinx-ai-index to sphinx docs builder

[Felix-Ruh]

This PR adds the sphinx-ai-index sphinx extension to the documentation extensions. This will automatically generate a ai_docs_index.json file, which serves as efficient retrieval interface for AI agents or other applications.
It also adds the ability to add an optional page-summary directive to a page, which will be added to this index, but will not be rendered into HTML.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.88%. Comparing base (4e10030) to head (b466240).
⚠️ Report is 260 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1671      +/-   ##
==========================================
+ Coverage   86.87%   88.88%   +2.00%     
==========================================
  Files          56       71      +15     
  Lines        6532    10015    +3483     
==========================================
+ Hits         5675     8902    +3227     
- Misses        857     1113     +256     

Flag	Coverage Δ
pytests	88.88% <ø> (+2.00%)	⬆️

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